Just add a script to the first conversation that starts another conversation
object oOwner = GetObjectByTag("conversation owner's tag");
DelayCommand(0.5, AssignCommand(oOwner, ActionStartConversation(GetFirstPC(), "conv. 2 filename", FALSE, CONVERSATION_TYPE_CINEMATIC, TRUE)));
Use DelayCommand to give the game time to close the first conversation and make sure that the first conversation ends to the node that runs the script. Ideally however you should combine the two dialogs into one file unless you want the second conversation happen in another location in the module and/or between other characters.